1. Field of the Invention
This invention relates to apparatuses and methods for measuring characteristics of optical fibers in length directions based on stimulated Brillouin scattering effects that occur in optical fibers.
2. Description of the Related Art
Recently, optical fibers are frequently used as information transmission media to secure high-speed transmission for large amounts of information. In order to secure satisfactory communication qualities, it is necessary to periodically perform measurements on characteristics of optical fibers in length directions. For example, measurements are performed to locate faults or defects that actually occur in optical fibers or that may likely occur. Specifically, OTDR (i.e., Optical Time Domain Reflectometer) measurement techniques are provided to measure characteristics (e.g., distortions) of optical fibers. That is, light pulses are input into one ends of optical fibers, wherein measurement is performed with respect to backward scattering light that occur in optical fibers during propagation of light pulses therethrough.
Since the OTDR measurement techniques are capable of specifying distorted positions of optical fibers, they are applicable to optical fiber sensors and the like that measure temperature distributions in environments for facilitating optical fibers as well as distributions of physical values such as distortions. In order to perform maintenance and management with respect to large-scale structures such as dams and embankments, it is necessary to detect distortions of large-scale structures. In this case, large-scale structures are wired with optical fibers whose characteristics such as distortions are measured by optical fiber sensors. Recently, it is strongly demanded to develop high-performance optical fiber sensors, having high spatial resolutions, which can specify distorted positions of optical fibers as accurately as possible.
To cope with the above demand, there is provided a measurement apparatus that performs measurement based on stimulated Brillouin scattering effects induced in xe2x80x98measuredxe2x80x99 optical fibers. Specifically, stimulated Brillouin scattering effects occur in optical fibers in which probe beams are input into one ends while pump beams are input into other ends. FIG. 5 is a block diagram showing an example of the measurement apparatus using stimulated Brillouin scattering effects. Herein, reference numeral 100 designates a light source that comprises a semiconductor laser 101 and a signal generation circuit 102. The signal generation circuit 102 performs frequency modulation or phase modulation on laser beams output from the semiconductor laser 101, thus generating modulation signals. Reasons why the frequency modulation or phase modulation is performed on laser beams output from the semiconductor laser 101 will be described later. Briefly speaking, however, the frequency modulation or phase modulation is required to determine positions of correlation peaks that can be clearly recognized between probe light L11 and pump light L12, which are input into a measured optical fiber 107 from different ends respectively. Reference numeral 103 designates an optical coupler or branch that provides two branches with respect to laser beams output from the light source 100.
That is, laser beams of the first branch from the optical branch 103 are input into a light modulator 104, wherein they are subjected to modulation to shift light frequencies thereof. Due to the modulation of the light modulator 104, sidebands are caused to occur with respect to the center wavelength of laser beams. The light modulator 104 comprises a microwave generator 105 and a light intensity modulator 106. The light modulator 104 modulates laser beams to produce sidebands in order to cause stimulated Brillouin scattering effects in the measured optical fiber 107. The microwave generator 105 generates microwaves for frequency shifting, which are imparted to laser beams output from the optical branch 103. The light intensity modulator 106 produces sidebands having frequency differences, which match frequencies of microwaves generated by the microwave generator 105, with respect to the center frequency of laser beams input thereto. Incidentally, the microwave generator 105 can vary the frequency of microwaves output therefrom. The light intensity modulator 106 outputs the probe light L11, which is input into one end of the measured optical fiber 107. Specifically, the lower sideband is used for the probe light L11.
The optical branch 103 also provides laser beams of the second branch, which are input to a light delay 108. That is, the light delay 108 delays incoming laser beams with respect to time in order to delay the pump light L12, which is input into the other end of the measured optical fiber 107. Due to the provision of the light delay 108, a prescribed delay time is set between the probe light L11 and the pump light L12. Delayed laser beams output from the light delay 108 are supplied to the other end of the measured optical fiber 107 via an optical branch 109 as the pump light L12.
The probe light L11 propagate through the measured optical fiber 107 from one end to the other end. The optical branch 109 branches off the output light of the measured optical fiber 107 having light frequency bands containing the frequency band of the probe light L11. The intensity of the probe light L11 may be influenced by stimulated Brillouin scattering effects that occur in the measured optical fiber 107. A light wavelength filter 110 has a filtering characteristic to allow transmission of only the lower sideband, within the light output from the optical branch 109, therethrough. A light detector 111 detects light power of the lower sideband that is isolated by the optical wavelength filter 110.
In the measurement apparatus having the aforementioned configuration shown in FIG. 5, laser beams that are subjected to frequency modulation or phase modulation and that are output from the light source 100 are supplied to the optical branch 103, which in turn provides laser beams of the first branch that are input into the light modulator 104. In the light modulator 104, laser beams are modulated (in intensity) to provide the probe light L11 whose light frequency can be varied. The probe light L11 is incident on one end of the measured optical fiber 107. In addition, the optical branch 103 provides laser beams of the second branch that are delayed by the prescribed delay time in the light delay 108 and that are then incident on the other end of the measured optical fiber 107 via the optical branch 109 as the pump light L12.
Both the probe light L11 and the pump light L12 are respectively produced based on the same laser beams that are modulated in frequency or phase in the same light source 100. Therefore, the probe light L11 and the pump light L12, which are input into the measured optical fiber 107 from opposite ends respectively, are mutually influenced by each other to periodically cause correlation peaks. At each position showing a correlation peak, a xe2x80x98constantxe2x80x99 light frequency difference appears between the probe light L11 and the pump light L12, which may be amplified in light intensity due to stimulated Brillouin scattering effects.
At other positions other than the positions of correlation peaks, the probe light L11 and the pump light L12 may be normally varied in light frequencies, so that the probe light L11 may not be affected by Brillouin amplification and will be substantially unchanged in light intensity. Therefore, it can be said that the gain of the probe light L11 may be greatly caused by Brillouin amplification at the positions of correlation peaks.
The probe light L11 whose gain is caused by Brillouin amplification is output from the other end of the measured optical fiber 107, from which it is supplied to the optical branch 109. Then, the probe light L11 that is transmitted through the optical branch 109 is input into the light wavelength filter 110, wherein a part of the probe light L11 corresponding to the lower sideband is isolated and is supplied to the light detector 111. Thus, the light detector 111 detects the intensity of the output light of the light wavelength filter 110.
FIGS. 6A and 6B diagrammatically show correlation peaks that appear in the measured optical fiber 107 shown in FIG. 5. In FIG. 6B, reference symbol xe2x80x98fmxe2x80x99 represents the frequency of the frequency modulation that is performed in the semiconductor laser 101, and xe2x80x98dmxe2x80x99 represents an interval of distance between adjoining correlation peaks. The following description is made with respect to the frequency modulation that is performed on laser beams radiated from the semiconductor laser 101, whereas in the case of the phase modulation that may be performed on laser beams radiated from the semiconductor laser 101, xe2x80x98fmxe2x80x99 should be read as the frequency of the phase modulation. As shown in FIG. 6B, the stimulated Brillouin scattering may intensely occur at the position of a correlation peak formed between the probe light L11 and the pump light L12, which are input into the measured optical fiber 107 from opposite ends. Herein, reference numerals 120, 121, and 122 designate waveform spikes representing correlation peaks, wherein the waveform spike 120 represents a zero-order correlation peak, the waveform spike 121 represents a first-order correlation peak, and the waveform spike 122 represents a second-order correlation peak. At the position of the zero-order correlation peak 120, the optical path difference between the probe light L11 and the pump light L12 becomes zero.
The distance dm between adjoining correlation peaks can be calculated using the frequency fm of the frequency modulation of the light source 100 and the light velocity v measured inside of the measured optical fiber 107 in accordance with the following equation (1).                               d          m                =                  v                      2            ·                          f              m                                                          (        1        )            
The equation (1) shows that the distance dm between adjoining correlation peaks depends upon the frequency fm of the frequency modulation that is performed on laser beams of the semiconductor laser 101.
FIGS. 7A to 7C show variations of positions of correlation peaks that are caused by varying the frequency of the frequency modulation. As shown in FIGS. 7B and 7C, when the frequency fm of the frequency modulation is varied, the distance dm between adjoining correlation peaks is correspondingly varied; thus, it is possible to change positions of correlation peaks. However, it is impossible to change the position of the zero-order correlation peak 120 by merely varying the frequency fm of the frequency modulation. Incidentally, reference symbol xcex4 represents spatial resolution of a correlation peak.
It was described above that the position of the zero-order correlation peak 120 matches the position at which the optical path difference between the probe light L11 and the pump light L12 becomes zero. Next, a method of changing the position of the zero-order correlation peak 120 will be described with reference to FIGS. 8A to 8C. That is, it is possible to change the position of the zero-order correlation peak 120 by varying the delay time of the light delay 108 shown in FIG. 5. Herein, the position of the zero-order correlation peak 120 does not depend upon the frequency fm of the frequency modulation. Therefore, by varying the delay time of the light delay 108, it is possible to easily move the position of the zero-order correlation peak 120 as well as the positions of the first-order correlation peak 121 and second-order correlation peak 122 without varying the distance dm between adjoining correlation peaks.
In the above, however, it may be meaningless that spatial resolutions of correlation peaks be greatly increased compared with the moving distances of the positions of the correlation peaks. The spatial resolution xcex4z can be calculated using the Brillouin gain linear width xcex94xcexdB, the frequency fm of the frequency modulation of the light source 100, the frequency variation xcex94f that occurs in the frequency modulation of the light source 100, and the light velocity v measured inside of the measured optical fiber 107 in accordance with the following equation (2).                               δ          z                =                              v            xc3x97                          ν              B                                            2            ⁢            π            xc3x97                          f              m                        xc3x97            Δ            ⁢                          xe2x80x83                        ⁢            f                                              (        2        )            
According to the above equation (2), it is necessary to adjust the spatial resolution xcex4z to be sufficiently small compared with the moving distance of the correlation peak while adequately adjusting the frequency fm of the frequency modulation of the light source 100. Details of this technology may be disclosed in Japanese Unexamined Patent Publication No. 2000-180265.
In the measurement apparatus of FIG. 5 using stimulated Brillouin scattering effects, both the probe light L11 and the pump light L12 are continuous light beams, correlation peaks of which may emerge periodically. In order to measure characteristics of the measured optical fiber 107, it is necessary to adjust the delay time of the light delay 108 and the frequency fm of the frequency modulation in such a way that a single correlation peak emerges in the measured optical fiber 107.
In principle, the measurement apparatus measures characteristics of the measured optical fiber at the position corresponding to the correlation peak. In order to perform measurement entirely over the measured optical fiber 107 in the length direction, the measurement apparatus should be adjusted in such a way that a single correlation peak exists in the measured optical fiber 107, and then it is moved from one end to the other end of the measured optical fiber 107. As described above, however, the measurement apparatus must deal with correlation peaks that periodically emerge in the measured optical fiber, wherein the measurement can be performed only for a small distance, which may be several meters or so, while securing high spatial resolutions. Hence, there is a problem that the measured optical fiber must be limited in length in the measurement.
It is an object of the invention to provide an apparatus and a method for measuring characteristics of optical fibers, wherein measurements can be reliably performed on optical fibers along relatively long lengths (or distances) while securing high spatial resolutions.
An optical fiber characteristic measurement apparatus of this invention includes a light source for producing laser beams that are subjected to frequency modulation, and a light modulator for modulating laser beams to produce sidebands with respect to the center wavelength of laser beams, so that the lower sideband of modulated laser beams is used as probe light (L1) input into one end of the measured optical fiber. A pulse modulator produces laser pulses based on laser beams as pump light (L2), which is input into the other end of the measured optical fiber. Thus, as the pump light propagates through the measured optical fiber, correlation peaks sequentially emerge at different positions along with the measured optical fiber.
The output light is extracted from the other end of the measured optical fiber and is supplied to a timing adjuster, which adjusts a transmission timing (T1) to allow transmission of light proximate to a measuring point in the measured optical fiber therethrough. That is, the light transmitted through the timing adjuster is supplied to a light detector via a light wavelength filter, wherein the intensity of the light proximate to the measuring point in the measured optical fiber can be accurately detected to determine characteristics of the measured optical fiber.
In the above, the frequency of the frequency modulation of the light source is slightly increased or decreased to move a correlation peak leftwards or rightwards in relation to the measuring point in the measured optical fiber.
Thus, it is possible to reliably measure characteristics of the measured optical fiber entirely over the relatively long distance with high spatial resolutions.